1. Field of the Invention
The present invention relates to an optical signal processing apparatus, an optical signal transmission system and an optical signal processing method, and, in particular, to an optical signal processing apparatus and an optical signal processing method applied to an optical signal transmission system in which influence of polarization mode dispersion of an optical fiber is mitigated by means of polarization scrambling, and the optical signal transmission system.
2. Description of the Related Art
Technology of mitigating influence of polarization mode dispersion (PMD) of an optical fiber which is a signal transmission medium for transmitting an optical signal, by polarization scrambling, is known (see the non-patent documents 1-4, mentioned below).
FIG. 1 (a) shows a block diagram illustrating an optical signal transmission system in such case.
In FIG. 1 (a), an optical signal transmitted from a transmission apparatus 10 undergoes polarization scrambling by a polarization scrambler 40, then, is transmitted by an optical fiber 50, and, is received by a reception apparatus 20. In the reception apparatus 20, transmitted information is extracted from the thus-received optical signal, and then, undergoes forward error correction (FEC) processing by a forward error correction processing part 25.
FIG. 1 (b) illustrates functions in this optical signal transmission system.
In FIG. 1 (b), a curve labeled by WITHOUT POLARIZATION SCRAMBLING shows penalty obtained when polarization scrambling by the polarization scrambler 40 is not carried out. A curve labeled by WITH POLARIZATION SCRAMBLING shows penalty obtained when polarization scrambling by the polarization scrambler 40 is carried out. A curve labeled by WITH POLARIZATION SCRAMBLING AND FEC shows a penalty for a case where polarization scrambling by the polarization scrambler 40 is carried out and also forward error correction processing is carried out on the reception side.
As can be seen from FIG. 1 (b), by means of the polarization scrambling, PMD (Polarization Mode Dispersion) tolerance can be improved, and, also, by means of the forward error correction processing on the reception side, the PMD tolerance can be further effectively improved.
The above-mentioned ‘penalty’ means an index indicating a degree of disadvantage in an optical signal transmission system due to polarization mode dispersion. In order to achieve proper optical signal transmission, this value should be controlled to be lower than a predetermined level. Further, the above-mentioned ‘PMD tolerance’ shows an allowable maximum value of a polarization mode dispersion amount for controlling the penalty to be lower than the predetermined value.
As known, polarization scrambling causes jitter having a value equal to differential group delay (DGD) in an optical signal transmitted.
FIG. 2 shows this phenomenon. FIG. 2 (a) shows an optical signal transmission system the same as that shown in FIG. 1 (a), and FIG. 2 (b) shows jitter occurring due to polarization scrambling in this system.
FIGS. 6-8 illustrate influence of polarization mode dispersion (PMD).
FIGS. 6-7 illustrate first-order PMD in a single-mode fiber.
FIG. 6 (a) shows a transmission signal, which is transmitted by an optical fiber as shown in FIG. 6 (b). As shown in FIG. 6 (b), in the single-mode fiber, two polarization modes having an angle θ therebetween exist, and signal transmission speeds are different between these polarization modes. Even in this case, as long as a signal is transmitted within a single polarization mode, as shown in FIG. 6 (c), (d), no influence of the polarization mode dispersion occurs.
On the other hand, when a signal is transmitted through the two polarization modes as shown in FIG. 7 (b), since the signal transmission speeds are different between these polarization modes, influence of the polarization mode dispersion occurs as shown in FIG. 7 (c), and thus, as shown in FIG. 7 (d), the signal waveform is distorted.
FIG. 8 shows a model assuming a state in which a plurality of optical fibers 50-1, 50-2, 50-3, . . . , are connected. In this case, when the respective optical fibers are connected where polarization modes thereof have different angles θ, differential group delay and the angle θ through all the optical fibers thus connected depend from a wavelength of a signal to transmit. In such a case, polarization mode dispersion, i.e., high-order polarization mode dispersion, occurs, which has complicated characteristics varying due to ambient temperature and so forth, and thus, it is not easy to estimate influence thereof to effectively compensate the same.
The above-mentioned polarization scrambling enables efficient error collection in forward error correction processing by changing a polarization state at a high speed with respect to a FEC frame period in the forward error correction processing so that all the polarization modes occur within a single FEC frame period (see the patent document 5 mentioned below).
However, when the polarization scrambling is thus carried out faster, jitter caused by the polarization scrambling becomes serious accordingly as known.
FIG. 3 illustrates jitter tolerance. In FIG. 3, an area enclosed by polygonal lines corresponds to the jitter tolerance. This area represents a range in which influence of the jitter is sufficiently small so that transmitted information can be extracted with high precision through clock recovery carried out on a received signal (see patent document 6 mentioned below).
As shown in FIG. 3, when a jitter frequency (horizontal axis) increases, the jitter tolerance decreases accordingly, positive capture of a signal through clock recovery becomes difficult, and thus, it is not possible to extract the transmitted signal from the received optical signal with high precision. That is, since the jitter frequency increases as the polarization scrambling is made faster, influence of the jitter amplitude on the jitter tolerance increases, and thus, it is not possible to extract the transmitted signal from the received optical signal with high precision.
FIG. 4 shows increase/decrease in the above-mentioned penalty with respect to the scrambling frequency in the polarization scrambling. As shown in FIG. 4, when the polarization scrambling is made slower (left hand of FIG. 4), the forward error correction processing cannot be carried out efficiently, and thus, the penalty increases. On the other hand, when the polarization scrambling is made faster, while the forward error correction processing can be carried out efficiently, the jitter tolerance lowers as described above with reference FIG. 3, whereby, extraction of the transmitted information cannot be carried out with high precision. Thus, the penalty also increases (solid curve in FIG. 4).
This is because, as a result of the polarization scrambling being carried out faster, influence of the jitter amplitude on the jitter tolerance increases as shown in FIG. 3, whereby clock recovery of the received signal cannot be carried out properly, and thus, it is difficult to extract the transmitted information with high precision.
Documents disclosing related arts are listed below:    Patent Document 1: WO2004/083945 A1;    Patent Document 2: 2000-33079;    Patent Document 3: 2004-219701;    Patent Document 4: 2005-65273;    Non-patent Document 1: “Multi-channel PMD Mitigation and Outage Reduction Though FEC With Sub-Burst-Error-Correction Period, PMD Scrambling”, Xiang Liu, IEEE member, Chongjin Xie, IEEE member, Adriaan J. van Wijngaaden, IEEE senior member, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, SEPTEMBER 2004, 2183-2185 pages;    Non-patent Document 2: “Multi-channel PMD Mitigation through forward error correction with distributed fast PMD scrambling”, X. Liu, C. Xie, Adriaan J. van Wijngaaden, WE2 1-3 pages;    Non-patent Document 3: “Improved PMD Tolerance in Systems Using Enhanced Forward error correction through Distributed Fast Polarization Scrambling”, X. Liu, C. R. Gites (1), X. Wei (2), A. J. van Wijngaaden (2), Y. H. Kao (3), C. Xie (1), L. Moller (1) ECOC 2005 Proceedings-Vol. 3, Paper Wel. 3.6, 343-344 pages;    Non-patent Document 4: “Direct Measure of System Margin Enhancement By Polarization Scrambling”, C. R. Davidson, H. Zhang, Y. Cai, L. Liu, J.-X. Cai, A. Philipatskii, M. Nissov, Neal S. Bergano, WE1;    Non-patent Document 5: “Experimental evaluation of the effect of polarization scrambling speed on the performance of PMD mitigation using FEC”, Zhihong Li, Jinyuu Mo, Yl Dong, Yixin Wang, Chao Lu, MF69;    Non-patent Document 6: “Jitter and wander tolerance of network interfaces”, ITU-T Rec. G. 8251 (11/2001), 5 and 8 pages;    Non-patent Document 7: “Present situation and problems of automatic polarization mode dispersion compensating technology”, Fujitsu Laboratories, Akihiko Isomura, Joji Ishikawa, OPTRONICS (2003), No. 10, 1-4 pages